1. Field of the Invention
This invention relates to an instantaneous gas water heater, and more particularly to an instantaneous gas water heater of the type in which a hot water outlet temperature is controlled in response to the volume of burner gas utilized therein.
2. Description of the Prior Art
A conventional instantaneous gas water heater typically contains a proportional valve which controls the volume of gas to be burned therein so as to maintain a constant hot water outlet temperature, a water flow switch therein being turned on or off in response to a differential pressure or to the flow rate of the water. However, this type of conventional instantaneous gas water heater typically exhibits the drawback in that the hot water outlet temperature decreases when the flow rate of water is increased beyond the point where the maximum flow rate of gas burned therein can heat the flowing water to the desired temperature, i.e., due to the physical characteristics of the instantaneous water heater. This disadvantage will be described with reference to FIG. 8.
FIG. 8 shows a characteristic plot of hot water temperature in a conventional instantaneous gas water heater, wherein the abscissa indicates the hot water flow rate Q and the ordinate indicates the hot water outlet temperature T. The dotted line b indicates the water inlet temperature and the solid line a indicates due capacity limitation curve of the water heater. It is common with conventional water heaters that the hot water outlet temperature is set at about 40.degree. C. and that the hot water is fed out directly from a faucet. As shown in FIG. 8, hot water of 40.degree. C. can be obtained when it is flowing at a supply rate less than Q.sub.1. When the hot water supply rate exceeds Q.sub.1, the hot water outlet temperature decreases along the line a, thereby resulting in a hot water outlet temperature of less than 40.degree. C. Therefore, as described above, even if a gas proportional valve is utilized in order to maintain a constant water temperature for practical convenience, its effect may be insufficient, resulting a lower hot water temperature than is desired.
The conventional type of water heater may experience the following disadvantages when operating under the following conditions:
1. the instantaneous gas water heater with proportional gas operation has a proportional range of 4 Gou to 16 Gou, (where, "Gou" is the Japanese conventional unit meaning output/min. of a water heater, 1 Gou is equal to 25 Kcal/min., therefore 4 Gou is 4.times.25-100 Kcal/min.)
2. the temperature setting of the water heater is 70.degree. C. for high temperature and 35.degree. C. for a low temperature;
3. the temperature of the inlet water is 5.degree. C. to 25.degree. C.;
4. the minimum flow rate of water required for operating a conventional type of water flow switch is approximately 3.0 liter/min., and further when
(a) the setting temperature is set at a high temperature of 70.degree. C. and the water inlet temperature is at 5.degree. C., 4 Gou will result in an output of 100 Kcal/min., resulting in the following equations:
______________________________________ (1) 100/(70 - 5) = 1.54 liter/min. (2) (70 - 5) .times. 3 = 195 Kcal/min. ______________________________________
The output/min. number, Gou, is calculated according to the equation Q.times.(Ts-Tc)/25=Gou, where Q is the flow rate, Ts is the setting (desired) temperature and Tc is the inlet water temperature. EQU 195/25=7.8 Gou (3)
From the equation 1, it is seen that under the above-noted conditions, irrespective of the fact that the water heater has the capability of igniting the burner below a flow rate of 1.54 liter/min., a conventional water heater has a non-usable area shown by the cross hatching in the graph of FIG. 11 since the minimum flow rate of water for operation of the switch is set at 3.0 liter/min. Thus, in practice, burner ignition can be made only in a proportional range of 7.8 Gou as indicated by equations 2 and 3. Therefore, it may occur that a small volume of hot water cannot be obtained or used during the winter season.
(b) when the setting temperature is set at a low temperature of 35.degree. C. and the water feeding temperature is 25.degree. C., the following equations can be attained:
______________________________________ (4) 100/(35 - 25) = 10 liter/min. (5) (35 - 25) .times. 3 = 30 Kcal/min. 100/3 = 33.3.degree. C. (6) 25 + 33.3 = 58.3.degree. C. ______________________________________
From equation 4, it is seen that a flow rate of 10 liter/min. is required in order to provide hot water at 35.degree. C. In practice, it is apparent from equations 5 and 6 that burner ignition takes place at a flow rate of 3.0 liter/min. Thus at this flow rate the hot water temperature will equal 58.3.degree. C., corresponding to a crossing point between the minimum capacity line of the water heater and the line of 3.0 liter/min. Therefore, when it is desired to use a small volume of tepid water in the summer, only hot water is discharged, potentially resulting in a hot water burn.
Further, a connection from a conventional water heater to a solar system generally cannot be made by a conventional water flow rate switch. Since a flow rate of water of more than 3.0 liter/min. effects burner ignition when inlet water of high temperature flows into the heater, the burner ignition is continued during a minimum level of inlet water even if its temperature is increased. The hot water is thus boiled, causing a safety device and operate to turn off the ignition. That is, when inlet water at 80.degree. C. flows into the water heater, a minimum proportional range is 4 Gou and the volume of hot water is 3 liter/min. The increase in temperature of 100 Kcal/3 liter/min. equals 33.3.degree. C. and results in a hot water outlet temperature of 80.degree.+33.3.degree.=113.3.degree. C.